I. Field
The present invention relates generally to communication, and more specifically to techniques for controlling transmit power used for a data transmission in a wireless communication system.
II. Background
In a wireless communication system, a wireless device (e.g., a cellular phone) communicates with one or more base stations via transmissions on the downlink and uplink. The downlink (or forward link) refers to the communication link from the base station to the wireless device, and the uplink (or reverse link) refers to the communication link from the wireless device to the base station. The downlink and uplink are allocated different frequency bands in a frequency division duplex (FDD) system.
In a Code Division Multiple Access (CDMA) system, which is an FDD system, a base station may simultaneously receive uplink transmissions from multiple wireless devices. Since these uplink transmissions occur over a shared frequency band, the uplink transmission from each wireless device acts as interference to the uplink transmissions from the other wireless devices. For each wireless device, the interference due to the other wireless devices degrades the received signal quality of the wireless device. The signal quality (denoted as “SIR”) may be quantified by a signal-to-noise-plus-interference ratio or some other quantity. Each wireless device needs a particular received SIR or better in order to achieve the desired level of performance, which may be quantified by a particular block error rate (BLER) or some other performance measurement.
To efficiently utilize the uplink capacity of a base station, an uplink power control mechanism is typically used to control the transmit power of each wireless device. This power control mechanism is normally implemented with two power control loops, which are often called an “inner” loop and an “outer” loop. The inner loop adjusts the transmit power of the wireless device such that its received SIR, as measured at the base station, is maintained at an SIR target. The outer loop adjusts the SIR target to maintain a BLER target, which may be, for example, 1% BLER. The wireless device transmits with excessive power, thereby causing too much interference, if the BLER is too low and provides unsatisfactory performance if the BLER is too high. The uplink power control mechanism thus attempts to reduce transmit power and interference while maintaining the desired link performance for the wireless device.
The uplink power control mechanism works well if transmit power control (TPC) commands sent by the base station to direct each wireless device to adjust its transmit power are both accurately generated by the base station and reliably received by the wireless device. However, either one, or both, of these premises may not be true for various reasons, as described below. When such is the case, a sudden and large change in transmit power (i.e., a “transmit power spike”) may occur for the wireless device. The transmit power spike may cause various deleterious effects. For example, a transmit power spike in an upward direction (i.e., an “upward” transmit power spike) may cause excessive interference to other wireless devices and reduced uplink capacity. A transmit power spike in a downward direction (i.e., a “downward” transmit power spike) may cause performance degradation of the affected wireless device and possibly a dropped call. There may also be a ripple or cascade effect whereby an upward transmit power spike in one wireless device causes interference and results in spikes in other wireless devices. All of these effects are undesirable.
There is therefore a need in the art for techniques to control the transmit power of a wireless device in a manner to mitigate the adverse effects of transmit power spikes.